What is a static relay?
You can find different types for different voltages and different powers.
In our case 12V 220W heatbed , you will need to use a DC-DC static relay, driven by 12V input voltage, and able to drive DC output power voltage.
This type of relay has MOSFET power transistor able to drive DC output voltage.
If you are using a 220V heatbed directly powered by your grid you will need to use a DC-AC static relay.
These have power triacs able to drive 220V alternative output voltages.
How to choose the power of your static relay?
The power your can draw out of a static relay depends on many factor. It’s type, it’s rated power, it’s ability to dissipate heat.
For DC-DC relays , They ofent get hot very easily, so take into account to always select one with 2 or 3 times it’s nominal load.
With a 220W 12V heatbed, the max current is around 18.3A.
- A 25A relay will be too small (max usable load would be 12A => 144W Max)
- A 40A relay will be just enough (2 times the nominal load) and might get hot
- A 60A relay ( able to support 3 times the nominal load) will be well adapted and should dissipate very little heat.
These have power tyristors or triacs.
For the 3D printer power range a simple 25A relay is enough for most usage.
If we take the Scalar XL with it’s 700W 220V heatbed,
Power(W) = Input Voltage(V) x Curent (A) x Cos Phy
Current= Power/ (Input Voltage x cos Phy)
If we take CosPhy = 0.6
Curent = 700W/(220V*0.6) => 5.8A MAX
This relay is 4.3 time more powerfull than it’s load.
Why a static relay?
With these powers, a static relay will protect you electronics from being damaged, and will also increase it’s lifepan.
If you are using Ramps boards with it’s Green power connectors, they can support only 11A.
Using more current is possible but you will need a very good cooling of the power components and of the power connector itself.
Heatbed Wiring using the Static relay.
Directly from your power supply
The +12V output de l’alimentation est relié directement au lit chauffant.
The heatbed output is then connectod to the « + » (pin 2) of the static relay
The « – » (pin 1) output is connected to the 0V of your power supply.
Pins 3 and 4 of the static relay are connected to D8 output of your Ramps board
Pay close attention to the polarity!
Between the Ramps and your static relay, you can use thin wires (24AWG for example) because very little power is transmitted to the static relay.
However, on your static relay output, make sure you are using proper wire diameter. (use 2.5mm² wires). The bigger the diameter, the lower the power loss, and your wiresd will stay cold.
Also attache the static relay on the aluminum extrusions.
For Scalar 3d Printers, you can attach it directly on the extrusion profiles. it will be greatly spread static relay heat.
With terminal strips
We will use terminal strip to connect with the available wires.
see above comments for more details.